The technical field of this invention concerns medical devices useful for the transmission of neural information from transected nerves to prosthetic apparatus, and methods for preparing and using such devices for controlling prosthetic apparatus.
When a peripheral nerve is transected (or severed) as a result of surgery or trauma, axons or specialized nerve cell appendages in the distal (the furthest away from the spinal cord) portion of the severed nerve degenerate and eventually die, leaving only the sheath in which they were contained. The axons in the proximal stump which are still connected to the spinal cord or dorsal root ganglion have the ability to regenerate provided that transection has not occurred too close to the nerve cell body. Factors in the degenerating distal stump appear to exert a positive influence on regeneration, a phenomenon termed "neurotropism". The absence of a distal stump in cases of amputation seems to inhibit nerve regeneration from the proximal stump.
The development of a technology suitable for the transmission of neural information from amputated limbs to prostheses has constituted a formidable problem. Although successful interfacing of the proximal stump of transected nerves with electrode structures has been accomplished in amphibians (see Marks, "Bullfrog Nerve Regeneration Into porous Implants", Vol. 163Anatom. Rec., p. 226 (1969)), with the first recordings taken from frog nerves by Maynard et al. in "Regeneration Electrode Units...", Vol. 183, Science, p. 547-549 (1974), reliable recordings from mammals using similar techniques have thus far not been obtained.
The unsuccessful or unsatisfactory mammalian results to date have been related, in part, to the use of extracorporeal devices employing gross electrodes. Another cause of the poor results appears to be due to poor alignment of the cut ends of the fascicles (nerve bundles within the nerve trunk) with the implanted microelectrodes. Attempts have been made to improve the electrical connection between the nerve and the electrode by suturing the nerve to the electrode, or by provision of an indirect contact via an electrical charge-conducting gel material. A gel-impregnated silicon transducer is disclosed, for example, in U.S. Pat. No. 4,623,355 issued to Sawruk on Nov. 18, 1986. Additionally, Edell, "A Peripheral Nerve Information Transducer...", Vol. BME-33, IEEE Transaction of Biomedical Engineering, pp. 203-214 (1986), has reported the use of a micromachined silicon device to extract nerve signals. Nonetheless, reliable devices for extracting neural signals from the nerves of amputated limbs in mammals heretofore have not been demonstrated.
An additional impediment to successful nerve-electrode connection is the trauma produced by the manipulation of the nerve end, and the subsequent suturing typically employed to maintain alignment. The trauma appears to stimulate the growth and/or migration of fibroblasts and other scar-forming connective tissue cells which prevent the regenerating axons in the proximal stump from reaching, making intimate contact with, or maintaining contact with the electrode.
In the area of nerve repair, (i.e., the regeneration of axons from the proximal stump to a distal stump) other approaches have been taken in attempts to lessen scar-forming trauma. These have included the use of nerve cuffs and channels to position the severed nerve and guide its regeneration. For example, Ducker et al. used silastic cuffs for nerve repair in Vol. 28, J. Neurosurg., pp. 582-587 (1968). Silicone rubber sheathing for nerve repair was reported by Midgley et al. in Vol. 19, Surgical Forum, pp. 519-528 (1968) and by Lundborg, et al. in Vol. 41, J. Neurophathol. Expt. Neurol., pp. 412-422 (1982). The use of bioresorbable polyglactin mesh tubing was reported by Molander et al. in Vol. 5, Muscle & Nerve, pp. 54-58 (1982), and the use of semipermeable acrylic copolymer tubes in nerve regeneration was disclosed by Uzman et al. in Vol. 9, J. Neurosci. Res., pp. 325-338 (1983).
However, the use of such nerve guidance materials can often add further problems. For example, some of the materials identified above have lead to inflammatory reactions in the test animals and/or have failed to exclude scar tissue formation within the channels. Moreover, the total number of axons, the number of myelinated axons, the thickness of the epineurium, and the fascicular organization of nerves regenerated within guidance channels are all typically less than satisfactory and compare poorly with the original nerve structure of the test animals.
There exists a need for better devices and methods for the transmission of neural signals from severed nerves in an amputated limb to a prosthesis. Devices and methods for the transmission of neural signals which would minimize surgical trauma, prevent interference with nerve growth by scar tissue, lessen immune responses, and improve the chances for successful regrowth of myelinated nerve and for maintenance of stable contact with the electrode would satisfy a long-felt need in this field.